Contributors shouldn't edit the CHANGELOG.md directly anymore

[dbeatty10]

resolves #237

Problem

We use automatically generated release notes to generate CHANGELOG entries now, so our contributing guide is out-of-date.

Solution

Bring the contributing guide up-to-date by updating the language and adding a hyperlink to the "automatically generated release notes" feature in GitHub.

Checklist

• This code is associated with an issue which has been triaged and accepted for development.
• I have read the contributing guide and understand what's expected of me
• [N/A] I have run this code in development and it appears to resolve the stated issue
• [N/A] This PR includes tests, or tests are not required/relevant for this PR
• [N/A] I have updated the README.md (if applicable)